In the definition of the of the sacraments, the Catechism of the Catholic Church, paragraph 1131 notes that the sacraments are “entrusted to the Church.” This entrustment means that it belongs to the Church to guard and offer the graces of the sacraments within her life and mission. It is through the sacrament of Holy Orders that Christ provides for the Church those who will carry out such guarding and offering of all the sacraments, such that His saving works can be made available to the whole world.
When considering the origins of Holy Orders, we look directly to Jesus and His own choosing of the apostles as sharers in His priesthood. Acknowledging Jesus Himself as the fulfillment of the Old Testament (which has its own priesthood), Jesus bestows upon the apostles a sharing in His eternal priesthood (offered in His one-time offering of Himself in death on the cross), that His saving sacrifice might be offered in and through them for the benefit of the whole Church. This sharing in His priesthood therefore is a call to serve so that the means of salvation (again, Jesus’s one-time sacrifice) might be made available to all through the Church.
In particular, we look at the Last Supper accounts to see how Jesus bestows this sharing in His priesthood (especially in John’s account) through such acts as washing of their feet (which harkens back to the washing that priests of the Old Testament were to undergo as part of ordination [see Leviticus 8:1-13, and particularly verse 6]), in instituting the Eucharist with the exhortation “Do this in memory of me,” and in his prayer over them that they be consecrated in the truth as found in John 17.
Early testimony of the Church shows that three levels or “degrees” of Holy Orders were established from the time of the Apostles. In the apostles themselves and in their immediate successors, the bishops, the fullness of Holy Orders resides. As co-workers with the bishops are the “presbyters” (what we familiarly call “priests,”) whose ordination is onto the offering of the sacrifice of the Eucharist in configuration onto Christ. Finally, deacons receive orders “onto service,” bestowing upon them a special relation to the bishop in the task of service to him and to the priests, along with service of the Word and charity.
Throughout the entire Church history these three degrees of Holy Orders have remained firmly in place within the hierarchy of the Church. Holy Orders is conferred by “ordination,” unto each degree, beginning with diaconate, followed by priesthood for many of these, from whom a few are called to be bishops – thus forming three degrees of the hierarchy). While other minor orders have also existed during the history of the Church (such as sub-deacon, acolyte, etc.), and though the order of the diaconate only recently has been renewed in the well-founded presence of men ordained onto diaconate so as to serve the Church permanently in the call of diaconal service, the three degrees have remained consistent. Each degree is conferred by its own particular ordination rite that is centered on the laying on of hands and the prayer over the candidate/s to be ordained which express a calling upon the Holy Spirit to come down on those ordained according to the degree to which they are receiving ordination. These details of the ordination rite, along with how the man is changed and configured onto Christ will be treated in the next installment.
For further reading: In the Catechism of the Catholic Church, the paragraphs 1536-1571 give a thorough overview of Holy Orders, including detailed description of each of the three degrees of orders.